One known method for exposing photographic materials, is an image formation method according to a so-called scanner system which comprises scanning an original drawing, and exposing a silver halide photographic material, based on its image signal, thereby forming a negative image or a positive image corresponding to an image of the original drawing.
Further, in the case of direct printing to a press plate, not passing through a dot to dot work process after output from a scanner to a film, or to a scanner light source having a soft beam profile, a scanner light-sensitive material having super hard characteristics has been desired.
A number of light-sensitive materials have been known which have light-sensitive layers on supports and perform image formation by image exposure. Of these, image formation by heat development is advantageous for environmental preservation and can simplify image formation means.
In recent years, it has been greatly desired to reduce the amount of processing waste fluid, from the viewpoints of environmental preservation and space saving in the field of photomechanical processes. Photothermographic materials for photomechanical processes have therefore been required which can be efficiently exposed with a laser scanner or a laser image setter and which can form sharp black images having high resolution. The photothermographic materials can dispense with the use of processing chemicals of the solution family, and so can provide to customers heat development systems which are simpler and do not damage the environment.
Methods for forming images by heat development are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, and D. Morgan and B. Shely, Thermally Processed Silver Systems A (Image Processing and Materials), Neblette, the eighth edition, edited by Sturge, V. Walworth and A. Shepp, page 2 (1969). Such light-sensitive materials contain reducible light-insensitive silver sources (for example, organic silver salts), catalytic active amounts of photocatalysts (for example, silver halides) and reducing agents for silver usually in the state where they are dispersed in organic binder matrixes. The light-sensitive materials are stable at room temperature. However, when they are heated at a high temperature (for example, 80.degree. C. or higher) after exposure, silver is produced by the oxidation-reduction reaction of the reducible silver sources which function as oxidizing agents with the reducing agents. The oxidation-reduction reaction is accelerated by the catalytic action of latent images generated by exposure. Silver produced by the reaction of the reducible silver salts in exposed areas provides black images, which make a contrast with unexposed areas to form images.
The photothermographic materials of this type have hitherto been known. However, many of these light-sensitive materials are applied in the form of coating solutions in which organic solvents such as toluene, methyl ethyl ketone (MEK) and methanol are used as solvents, thereby forming light-sensitive layers. The use of organic solvents as solvents is disadvantageous in terms of not only adverse effects to the human body in manufacturing processes, but also because of a rise in cost due to the recovery of the solvents and the like.
Methods for forming light-sensitive layers by use of aqueous coating solutions having no concerns for such disadvantages (hereinafter also referred to as "aqueous light-sensitive layers") have been proposed. For example, the use of gelatin as a binder is described in JP-A-49-52626 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-53-116144. Further, the use of polyvinyl alcohol as a binder is described in JP-A-50-151138.
Furthermore, an example in which gelatin is used in combination with polyvinyl alcohol is described in JP-A-60-61747. As another example, a light-sensitive layer in which water-soluble polyvinyl alcohol is used as a binder is described in JP-A-58-28737.
Surely, the use of such binders permits the formation of light-sensitive layers using aqueous coating solutions, resulting in high environmental and cost merit.
However, when polymers such as gelatin, polyvinyl alcohol and water-soluble polyacetals are used as binders, not only the compatibility with organic silver salts is poor, which causes a failure to obtain coated products fit for practical applications in respect to the quality of coated surfaces, but also the color tone of silver in developed areas turns brown or yellow, far from black which is considered to be inherently preferred, or the optical density of exposed areas is low and the density of unexposed areas is high, resulting in significant deterioration of commodity value.
Accordingly, techniques have been desired which provide photothermographic materials, aqueous light-sensitive materials excellent in environmental preservation and cost, good in the quality of coated surfaces, showing good color tone of silver in development, and having sufficient photographic characteristics.
Further, the application of techniques used in previously widely known silver halide photographic materials has become possible by the use of the above-mentioned aqueous light-sensitive materials. That is to say, it becomes possible to previously prepare light-sensitive silver halide emulsions having desired characteristics and to mix the resulting emulsions with organic silver salts, which can substantially improve the degree of freedom of design, compared with the preparation of light-sensitive silver halide emulsions by the organic solvent system.
European Patent 762,196 and JP-A-9-90550 disclose that high contrast photographic characteristics can be obtained by adding the group VII or VIII metallic ions or metallic complex ions to light-sensitive silver halide grains used in heat developable image recording materials, and adding hydrazine derivatives to the light-sensitive materials. However, in all of them, silver bromide is used as the light-sensitive silver halide, so that they have the problem that the Dmin in the UV region, the wavelength of a light source used in printing to a press plate, is too high to use for plate making. The Dmin in the UV region can be reduced by increasing the content of silver chloride. However, this has raised the problems that fog is liable to occur, that no hard image is obtained, and that it is difficult to obtain high Dmax.